A persistent problem associated with the use of metallic stenting is found in the formation of scar tissue coating of the vascularly located stent, the so-called process of restenosis. Moreover, metallic or polymeric non-absorbable stents may prevent vascular lumen remodeling and expansion. Numerous approaches have been tried to prevent or heal tissue injury and reduce complement activation of the immune response. Furthermore, there is a need for a reduced inflammatory response and lower potential for trauma upon break-up of an implant and/or its component materials. A desirable improvement target may be found in the need for increased flexibility of shape and structure of medical devices for implantation, particularly into blood vessels.
Among the many commercially available bioabsorbable polymers are poly-alpha-esters (e.g., lactides (i.e., L-lactide and D,L-lactide)) and glycolides, polyester ethers (i.e. polydioxanone), and polycarbonates (i.e., glycolide or lactide-co-trimethylene carbonate), and tyrosine based polycarbonates. Many other bioabsorbable polymers are being developed for commercial use, particularly in different modes of drug delivery, which polymeric substances include polyethylene glycol-co-lactides, polyanhydides, polyorthoesters, polyester-amides or cyanoacrylates.
The present inventors have recognized a need to develop a compatible polymer blend for implants, such as stents and vascular synthetic grafts, which provide a toughening mechanism to the base polymer when the medical device is deployed in the body. They have hypothesized that the later may be performed by imparting additional molecular free volume to the base polymer to encourage sufficient molecular motion to allow for re-crystallization to occur at physiological conditions especially when additional molecular strain is imparted to the implant. They have theorized that increased molecular free volume can also increase the rate of water uptake adding both a plasticizing effect as well as increasing the bulk degradation kinetics.